If you got here via link in Premiere Composer's error dialog, please try the troubleshooting in this article.
Premiere Composer sometimes may throw an unexpected error dialog for unknown reasons. That may be due to various bugs in Premiere Pro. In that case, there's a little or sometimes even nothing at all that we can do about it. We're sorry about it! Still, there are the following things you can try to fix it.
Troubleshooting method 1
Please try to restart Premiere Pro.
This method might work because Premiere cleans up its internal state when it's launched.
Troubleshooting method 2
Please try to reinstall Premiere Composer plug-in.
It's easy. Just open the Mister Horse Product Manager (you can access it from Premiere Composer's menu by selecting "Manage Products") and click the "Uninstall" button in the list. After it's done, the button will change to "Install." Click it to install the plug-in again.
This method might work because reinstallation fixes plug-in itself and it's dependencies.
We know this might feel like the infamous IT Crowd suggestion, "Have you tried to turn it off and on again?". But in most cases, it really works. Magic.